While the role of dopamine in positive symptoms of schizophrenia (SCZ) is well established, it is increasingly recognized that dopamine may only be a final common pathway of different etiological factors. Several lines of evidence point towards a role for neuroinflammation in the pathogenesis of SCZ. Neuroinflammation is also possibly related to brain atrophy, which is a consistent finding in SCZ. Positron emission tomography (PET) of translocator protein 18Kd (TSPO), a microglial mitochondrial protein provides an opportunity to study microglia activity, a marker of neuroinflammation, in-vivo. Previous studies used [11C]-PK1195, a radiotracer with many deficiencies as a PET radiotracer, and only in treated patients with SCZ. However, the role of microglia activation/neuroinflammation in antipsychotic nave SCZ, its relation to brain atrophy and clinical symptoms has not been examined using PET. Importantly, examination of neuroinflammation in Clinical high risk (CHR) subjects, which provides an opportunity for early intervention and possibly better outcome, has never been undertaken. Thus, the proposed study aims to obtain first in-vivo imaging data using a high-resolution (HRRT) scanner to investigate whether SCZ related disorders are associated with increased neuroinflammation by measuring [18F]-FEPPA binding in three groups of individuals (36 in each): antipsychotic nave SCZ patients, CHR and matched healthy volunteers (HV). [18F]-FEPPA is a novel radiotracer with desirable properties developed at Centre for addiction and mental health (CAMH), Toronto. Subjects will be scheduled for one PET scan and MRI scan each. The main objective is to test whether there is a significant effect of group (SCZ, CHR and HV) on [18F]-FEPPA binding in hippocampus and dorsolateral prefrontal cortex (DLPFC), while controlling for genotype. On Post-hoc analysis, we hypothesize that both antipsychotic nave SCZ and CHR will have higher [18F]-FEPPA binding in hippocampus and DLPFC, as compared to HV. This study addresses a relevant question in schizophrenia research; the role of neuroinflammation in the pathophysiology of SCZ, without the confound of antipsychotic medications, using state of the art imaging technology and second generation TSPO radioligands while controlling for genotype. Understanding the neurobiological changes associated with microglia activation has the potential to identify novel treatment targets (i.e. decrease neuroinflammation) in SCZ and in those at clinical high risk for the disease. There is increasing recognition that early identification and intervention is critical for better outcome in SCZ. By examining CHR and antipsychotic nave SCZ, potential early treatments can be conceived that could have significant impact on final outcome of SCZ, and even delay or abort its occurrence.